1. Field of the Invention
This invention relates to a method of image formation and, more specifically, to a method of forming extremely high contrast negative photographic images.
2. Description of the Prior Art
A method of obtaining photographic characteristics of a high contrast negative image by adding a hydrazine compound to a silver halide photographic emulsion is described in U.S. Pat. No. 2,419,975. U.S. Pat. No. 2,419,975 discloses that extremely high contrast photographic characteristics, such as a gamma (.gamma.) of more than 10, can be obtained by adding a hydrazine compound to a silver chlorobromide emulsion and developing the emulsion with a developer having a pH as high as 12.8. However, strongly alkaline developers having a pH near 13 are so unstable that they tend to be oxidized by air and, therefore, cannot be used or stored for long periods of time. Moreover, development at such a high pH tends to cause fog.
U.S. Pat. No. 3,386,831 describes a process for stabilizing an emulsion by adding a mono-phenylhydrazide of an aliphatic carboxylic acid into an essentially surface-sensitive photographic silver halide emulsion. The object and effect of the invention disclosed in U.S. Pat. No. 3,386,831 is to stabilize the emulsion and it differs from the object and effect of the present invention.
Ultra high contrast photographic characteristics, either of a negative image or of a positive image, are very useful for the photographic reproduction of an image of a continuous tone comprising a dot image which is useful in making printing plates or the reproduction of a line image. For the above purposes, hitherto a method of using a silver chlorobromide photographic emulsion having a silver chloride content of more than 50 mol%, preferably more than 75 mol%, and developing the emulsion with a hydroquinone developer having an extremely reduced effective concentration of sulfite ions (usually less than 0.1 mol/l) has been generally adopted. However, in this method, since the sulfite ion in the developer is present at a low concentration, the developer is very unstable and cannot be stored for a period exceeding 3 days. Furthermore, since a silver chlorobromide emulsion containing a relatively high percentage of silver chloride must be used, high sensitivity cannot be obtained.
Accordingly, use of an emulsion of high sensitivity and a stable developer to obtain ultra high contrast photographic characteristics useful for the reproduction of a dot image or a line image have been strongly desired.